1. Field of the Invention
This invention relates to the field of fluid flow velocity measurement. In particular this invention relates to a method for measuring the vortex shedding frequency to accurately measure low velocities of fluid flows.
2. Description of the Relevant Art
Electronic package designers need to characterize the performance of their products in many ways. One such characterization is in the form of heat transfer as a function of air velocity. The thermal performance of packaging has become an increasingly important issue in recent years. The high performance device market has created a demand for increasing chip densities and increasing chip complexities. Both properties lead to increased power consumption, which in turn leads to increased heat dissipation. The heat transfer characterization typically involves measurement of air velocities from 0 m/s to 3 m/s. Air velocity measurement accuracy is particularly important in the velocity range between 0.25 m/s and 1 m/s, since this is the range of most customer applications.
High accuracy velocity measurement instruments are available. One such instrument uses Digital Particle Image Velocimetry (DPIV), in which particles in the flow stream are tracked and their velocities determined with digital image processing. Another such instrument uses Laser Doppler Velocimetry (LDV), in which the Doppler shift induced on a laser beam passing through the flow stream is determined. Instruments of these types are very expensive.
Inexpensive velocity measurement instruments are available. Examples of these instruments include the Venturi meter, the Pitot tube, and the Pitot-static tube. These instruments rely on the use of Bernoulli's equation to determine fluid flow velocity. At low velocities, the accuracy of these instruments suffers due to the development of boundary layers around the instruments. As the velocity decreases, the measurement error increases exponentially, and the uncertainty cannot be determined. For a typical Pitot-static probe with a 0.25 mm hole in an air stream at standard conditions, the uncertainty of the velocity measurement becomes indeterminate below 1.73 m/s.
Hence, a need exists for a method of fluid flow velocity measurement which is both accurate at low velocities and is inexpensive to implement.